The present invention relates to temperature sensors having a resistance whose value varies with respect to the temperature and more particularly to those of these sensors which can be used from a few K up to the usual atmospheric temperatures.
Numerous temperature sensors are already known capable of being used at cryogenic temperatures. In particular, carbon resistors are used but their range of use is limited (about 1.5 to 30 K) and their sensitivity varies greatly in this range, which prohibits the use of one and the same sensor to cover the whole of this range. Germanium resistors can be used between some 0.1 K and 100 K, but their sensitivity is much variable in that temperature range, so that again it is necessary to use several sensors successively to obtain measurements throughout the range. Moreover, their thermal inertia is fairly high, they are sensitive to magnetic fields and their cost is high. Temperatures between about 20 and 300 K can be measured with platinum resistors. But their sensitivity is low and those probes which ensure a good reproductibility of results are costly. Finally, semi-conductor junction diodes (particularly gallium arsenide) can be used between about 2 K and 300 K. But their sensitivity is low and the law of variation of the resistance with respect to the temperature is complex. Their cost is high and they are sensitive to magnetic field.
Ceramic resistors have already been described which consist of a sintered mixture of niobium nitride and niobium carbide. Such resistors have been described as having a very small temperature coefficient (Chemical Abstracts, Vol. 81, No. 2, July 15, 1974, abstract No. 7596e) and consequently are not suitable for use as temperature sensors.
Last, the prior art teaches preparation of thin layers of oxides such as vanadium oxide having a low electric resistance on shapphire polycrystalline alumina, beryllium oxide or glass (French patent specification No. 2 219 606, corresponding to U.S. Ser. No. 335,651 of H. Keith Eastwood et al).
It is an object of the invention to provide an improved temperature sensor which can be used over a wide range of temperatures and which is not substantially sensitive to the magnetic field, whose cost and thermal inertia are low. It is another object of the invention to provide a temperature sensor whose sensitivity varies with temperature according to a relation which may be represented by a simple mathematic formula.
According to an aspect of the invention, a temperature sensor comprises a thin layer of niobium nitride with a disordered or disorganized structure deposited on a substrate which has a high thermal conductivity and is electrically insulating at the operational temperatures, said layer being provided with terminal electrical contacts.
The thickness of the layer will typically be between 1000 Angstrom and a few thousand Angstrom. The substrate will be chosen according to the range of temperature envisaged.
When the sensor is to be used in the range of cryogenic temperatures, a strip of synthetic sapphire can be advantageously used as substrate. At low temperatures, sapphire has a thermal conductivity of the same order as that of metals currently considered as excellent heat conductors. Beryllium oxide can also be used. The favourable properties of these different materials appear from a comparison of their thermal conductivity values at 4.2 K, which are as follows:
______________________________________ copper of high purity 70 W.cm.sup.-1 . K.sup.-1 ordinary copper 3 W.cm.sup.-1 . K.sup.-1 sapphire (Al.sub.2 O.sub.3) 2 W.cm.sup.-1 . K.sup.-1 beryllium oxide intermediate between sapphire and copper ______________________________________
Sapphire also has the advantage of retaining at ambiant atmospheric temperatures an acceptable thermal conductivity of about 0.2 W.cm.sup.-1.K.sup.-1, which can be compared to that of copper, of about 5 W.cm.sup.-1.K.sup.-1.
Any other material may be used which is a good electrical insulator, has a sufficient thermal conductivity and provides a substrate bondable to the layer. Among additional materials which may be envisaged, loaded glasses can be cited.
According to another aspect of the invention, a process for manufacturing a temperature sensor includes the step of depositing a thin layer of niobium nitride by reactive cathode sputtering on an electrically insulating substrate and providing it with thermal contacts.
The layer thus obtained has a resistance R whose variation with respect to the absolute temperature T can in most cases be represented with sufficient accuracy by a relation of the type: EQU R(T) = R.sub.o exp (T/T.sub.o).sup.-1/4
over a wide range of temperatures.
The constant values R.sub.o and T.sub.o depend on the conditions of preparation, in particular on the temperature of the substrate at the time of deposition. In general, the deposition temperature is selected to obtain a compromise between a value which corresponds to a high sensitivity of the sensor (i.e. a rapid variation of the resistance with respect to the temperature), but at the cost of a high value of R.sub.o, and a deposition temperature which corresponds to a lower resistance R.sub.o, which makes the sensor less sensitive to outside interference.
Whatever the deposition conditions, a layer is obtained whose resistance R varies with temperature T according to a simple mathematical law over a wide range of temperatures. Over the whole of this range a "secondary" thermometer (i.e. a thermometer which is calibrated by reference with a standard thermometer) incorporating a sensor of the above type can be calibrated in a simple way for direct reading of the temperature by interpolation. This has a favourable effect on the cost, not only because of the economical character of the sensor itself but also because of the simplification of the device as a whole.
The invention will be better understood from a consideration of the following description of one embodiment of the invention.